This invention relates to a device for sensing the oxygen concentration in an exhaust gas, which is used to control the supply of fuel to an internal combustion engine.
Generally, when an internal combustion engine is being driven at the stoichiometrical air-fuel ratio, the amount of harmful components in the exhaust gas extremely decreases. In order to avoid air contamination, therefore, it is desirable that the internal combustion engine be driven at the stoichiometrical air-fuel ratio under any operation mode. However, this would increase the consumption of fuel. At the time of acceleration drive or high load drive, the air-fuel mixture is made rich, while at the time of normal drive or partial load drive it is made lean. Therefore, if the amount of fuel supplied to the internal combustion engine is so adjusted or controlled that at the time of high load drive, the engine may be driven at the stoichiometrical air-fuel ratio and that when the engine is used a lean mixture condition at the time of partial load drive, the amount of supplied fuel may be adjusted to an ideal one in accordance with the concentration of oxygen residual in the exhaust gas. Thus, the countermeasure against the exhaust gas and the saving of the fuel will be realized.
There has been developed a control means which, when the engine is driven, in at a high-load and a partial-load, senses the amount of oxygen in the exhaust gas and controls the amount of fuel injected from a fuel injecting device in correspondence to the amount of oxygen gas thus sensed. As the elements of this control means, two types of sensors have been developed, one of which is a stoichiometrical air-fuel ratio sensor designed to sense under a high load drive mode whether or not the oxygen concentration in the exhaust gas corresponds to the oxygen concentration of the air-fuel mixture at the stoichiometrical air-fuel ratio, and the other of which is a lean sensor designed to sense the oxygen concentration in the exhaust gas under the partial load drive mode or under lean mixture mode.
The stoichiometrical air-fuel ratio sensor has an oxygen concentration sensing element made of oxygen-ion permeable metal oxide and producing an electromotive force in correspondence to the oxygen concentration in the exhaust gas, said electromotive force being delivered from two electrodes made of, for example, platinum and provided on the sensing element. This electromotive force is suddenly changed when the stoichiometrical air-fuel ratio is attained, the driving mode corresponding to the stoichiometrical air-fuel ratio is achieved by increasing or decreasing the amount of fuel supplied to the engine until said electromotive force indicates such a sudden change.
The lean sensor comprises a solid electrolyte element made of oxygen-ion permeable metal oxide. The element is provided with a pair of porous electrodes on both surfaces. Oxygen in the exhaust gas is ionized by applying a voltage between the electrodes so that the resulting ions are diffused from one electrode to the other.
Generally, when the voltage applied between both electrodes is varied, the amount of current flowing between them is also varied. At the time of an optimum air-fuel ratio under a lean mode, however, if the solid electrolyte element is at a fixed temperature, the amount of current ceases to vary even if the applied voltage is varied. The unvaried current is hereinafter referred to as "a saturated current". Under a lean mode, therefore, an optimum air-fuel ratio is achieved by increasing or decreasing the amount of fuel supplied to the engine until said saturated current flows between the electrodes.
Since the value of the above-mentioned saturated current varies with a temperature, it is necessary to keep the temperature to be a fixed value. To this end, while checking a temperature sensor is used to control a heater so that the solid electrolyte element is kept at a constant temperature within the range of, for example, 700.degree. to 750.degree. C.
When it is attempted, under such circumstances, to control the supply of fuel to the internal combustion engine throughout one entire operation including any type of driving modes, it becomes necessary to use the stoichiometrical air-fuel ratio sensor, the lean sensor, the heater for the lean sensor and the temperature sensor for the heater. In the prior art, however, those sensors are separately prepared, or only the lean and temperature sensors are assembled together. In the prior art, therefore, in controlling the air-fuel ratio throughout the entire operation range of the internal combustion engine it is necessary to connect a plurality of such independent sensors to, for example, the exhaust pipe of the engine. This results in an increase in number of parts as well as in number of the assembling processes. Thus, the prior art has the disadvantages that such factors increases the cost.